1. Field of the Invention
The field of the present invention is that of optical filters disposed in optical waveguides.
2. Description of the Related Art
Optical filters, such as Bragg gratings written optically into optical waveguides, are finding increasingly important applications in optical systems, in which they are used for applications such as wavelength selection, gain equalization, and chromatic dispersion compensation, for example.
A waveguide conventionally comprises an optical core, the function of which is to transmit and possibly to amplify an optical signal, surrounded by optical cladding, the function of which is to confine the optical signal within the core. To this end, the refractive index n1 of the core and the refractive index n2 of the cladding are such that n1>n2. Thus the core and the cladding form a waveguide. As is well known in the art, the propagation of an optical signal in a monomode waveguide is divided between a fundamental mode guided in the core and secondary modes guided over a certain distance in the core+optical cladding combination, the modes also being known as cladding modes. The cladding is itself surrounded by an external medium whose refractive index n3 is either greater than or less than n2. The core+cladding combination associated with the external medium also forms a waveguide. A waveguide may consist of an optical fiber or a planar guide.
The core and/or the cladding of the guide may be doped, for example with germanium (Ge), to render it photosensitive for writing a Bragg grating. A Bragg grating conventionally comprises a periodic disturbance to the refractive index in the guide, which forms an index grating. This index disturbance is obtained by localized irradiation of the guide through a phase mask which determines the period Λ of the grating. The intensity and the modulation of the irradiation during writing define a Bragg grating characterized by the Bragg relation λB=2Λ·neff, where Λ is the period of the grating and neff is the effective index of the fundamental mode guided in the core of the optical fiber.
This relation characterizes a short-period Bragg grating. A short-period grating perpendicular to the axis of the fiber constitutes a reflective spectral filter centered on the Bragg wavelength λB, conventionally used in filtering applications or in multiplexing applications for adding or dropping a transmission wavelength. The Bragg gratings are associated with one or more components with a plurality of input-output channels, for example optical circulators or couplers.
Numerous forms of Bragg grating have been developed. Thus chirped gratings, in which the period Λ varies along the grating, are conventionally used in chromatic dispersion compensation applications.
Slanted Bragg gratings (SBG) and long-period gratings (LPG) are conventionally used in gain equalization applications. Such gratings are designed to enable coupling of the fundamental mode into the cladding modes, enabling filtering without reflection at the coupling wavelengths and eliminating the need for optical isolators.